wpa_supplicant:  Allow configuring scan frequencies.

This allows one to limit the channels that wpa_supplicant will
scan.  This is a useful addition to the freq_list configurable
in the network {} section.

Signed-hostap: Ben Greear <greearb@candelatech.com>
wpa_supplicant:  Allow vifs to scan only current channel.

If a VIF is already associated, then only scan on the associated
frequency if user requests such.  This is a big help when using
lots of virtual stations.

Signed-hostap: Ben Greear <greearb@candelatech.com>